1. Field of the Invention
The invention relates to a method for destroying a target formed by soft structures localized within a medium which prevents having access thereto directly by mechanical means.
Among the applications of such a method may be mentioned the destruction of living cells, e.g. tumoral cells inside the body of a patient, or the coloring of a material through its mass by causing microballs to explode and release dyes, or the localized use of a chemical reaction, by releasing material capable of reacting and contained in microballs.
The destruction by extracorporeal means of a target formed by mechanically resilient structures takes place at the present time using positive pressure elastic waves which subject said structures to compression forces.
By way of example, in known lithotriptors, elastic pulses of a duration of about a microsecond, focussed on a calculus pulverize the latter.
More particularly, in lithotriptors with an emitting source distributed over a surface and of piezoelectric type, these elastic waves, unless very specific arrangements are made, comprise positive pressure half waves (with respect to the mean pressure of the oscillation) and negative half waves. However, it is known that the latter do not contribute to the destruction of the calculi and have on the contrary the harmful effect of causing the formation of a barrier formed of bubbles generated by cavitation and which hinders the propagation of the beam, even interrupting it.
2. Description of the Prior Art
The prior art relative to the destruction of soft targets is formed, for example, by hyperthermia, i.e. localized heating at well defined temperatures. For this, trains of elastic waves are used focussed on the target. These wave trains may be generated (as in lithotripsy) by an emitting source distributed over a surface and of piezoelectric type but, whereas in lithotripsy pulses are generally used of very short duration (of about 1 .mu.s) and considerable peak powers (of the order of several tens of kilowatts), in hyperthermia the powers are reduced to peak values of 10 to 100 watts, for example, and the wave trains are emitted over much longer periods (reaching a second for example) sufficient to cause heating. In the case of hyperthermia, the frequencies are of the order of 0.5 to 5 MHz.